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AFE design issues for RTD ratiometric measurements

Posted: 04 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Sigma-Delta ADCs? resistance temperature detectors? RTDs? ratiometric?

Many system designers employ Sigma-Delta ADCs together with resistance temperature detectors (RTDs) for temperature measurements, but have difficulties with achieving the high performance as specified by the datasheet of the ADC they are using. For example, some designers may only be able to get 12-13 noise-free bits from a 16-18 bit ADC. The front-end techniques introduced in this article will enable designers to achieve 16+ noise-free bits in their system designs.

Using RTDs in a ratiometric measurement has the advantage in that it eliminates sources of error such as the accuracy and drift of the excitation current source. Below is a typical circuit for a 4-wire RTD ratiometric measurement circuit. The 4-wire configuration has the advantage that the error due to lead resistance can be cancelled.

Figure 1: Four wire RTD Ratiometric Measurement Circuit.

From the circuit above, we can derive the following two equations:

The general expression used to calculate the RTD resistance (RRTD ) when the ADC is operating in bipolar differential mode is given by:

Where:

CodeRTD is the ADC code.

CodeADC_Fullscale is the ADC full scale code.

The measured resistance value of the RTD is theoretically only related to the precision and drift of the reference resistor. Normally RRef is an accurate and low drift resistor with 0.1% precision.

When engineers design their products using this type of circuit, they will add some resistors and capacitors before the analogue input, external reference pins for low pass filtering, and over-voltage protection as shown in figure 2. In this article, we will show what should be considered in choosing suitable resistors and capacitors for better noise performance.

Figure 2: Typical 4-wire RTD Ratiometric Measurement circuit.


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